In an internal combustion engine, it is known for a fuel pump to supply fuel to a high-pressure accumulator (or common rail), from which it is delivered into each cylinder of the engine by means of a dedicated fuel injector. Typically, a fuel injector has an injection nozzle which is received within a bore provided in a cylinder head of the cylinder; and a valve needle which is actuated to control the release of high-pressure fuel into the cylinder from spray holes provided in the nozzle.
It is known to provide a fuel injector for an automotive engine with a piezoelectric actuator for controlling the injection of fuel into the engine. A piezoelectric actuator of the type used in a fuel injector includes a stack of piezoelectric layers (or elements) that are separated by a plurality of internal electrodes. The actuator includes positive and negative external electrodes that respectively connect to alternate internal electrodes, such that positive and negative internal electrodes interdigitate along the length of the piezoelectric stack, with a layer of the piezoelectric material in-between.
In use, an electrical connector having positive and negative terminals provides a voltage to respective positive and negative external electrodes, which produces an intermittent electric field between adjacent interdigitated electrodes. The intermittent electric field can be rapidly varied with respect to its strength, which in turn causes the stack of piezoelectric layers to extend and contract along the direction of the applied field.
Within a fuel injector, the lowermost piezoelectric layer of the stack is adjacent to a lower end cap and the uppermost piezoelectric layer of the stack is adjacent to an upper end cap. The lower end cap is coupled to an injector valve needle, either directly or through an intermediate mechanical and/or hydraulic coupling. In this way, the piezoelectric actuator is arranged to control movement of an injector valve needle towards and away from a valve seating, so as to control the injection of fuel. Advantageously, piezoelectric actuators thus allow accurate control over the amount of valve needle movement towards and away from the valve needle seat and, hence, over the rate of fuel injection.
Piezoelectric injectors come in both “de-energise to inject” (e.g. EP 0995901; EP 1174615) and “energise to inject” (e.g. EP 1555427) varieties. By way of example, in a de-energise to inject fuel injector, such as that described in EP 1174615, as the piezoelectric stack extends and contracts upon application and removal of the electric field, respectively, the injector valve needle is similarly caused to move away and towards the injector valve seat.
In use, fuel is prevented from being injected into an associated engine cylinder when the injector valve needle is securely engaged with its injector valve seat. In a de-energise-to-inject injector, such as that in EP 1174615, this is achieved by applying a voltage of approximately 250 V to the positive internal electrodes and approximately 50 V to the negative internal electrodes to give a differential voltage (or potential difference) of approximately 200 V across the electrodes. This level of differential voltage causes an appropriate extension of the piezoelectric stack, such that the injector valve needle remains in contact with the injector valve seat. Since fuel injection events are typically short in relation to the operating cycle of a fuel injector, the fuel injector needle is engaged with its associated valve seat for approximately 95% of the operating cycle of the fuel injector.
To inject fuel into the cylinder the differential voltage across the positive and negative internal electrodes is rapidly reduced, causing the piezoelectric stack to contract. The pressure of the fuel and the amount of fuel that it is intended to inject determines the required level of voltage reduction. For example, at a minimum fuel pressure of around 200 bar (such as when the engine is idling), the voltage applied to the positive internal electrodes may be reduced to approximately 20 V; while at a maximum pressure of around 2000 bar, the voltage applied to the positive internal electrodes may be reduced to approximately −20 V, briefly making the positive internal electrodes negative (i.e. a bipolar mode of operation).
An significant advantage of this type of injector drive system is that bipolar operation, in which the voltage on the piezoelectric actuator is allowed to go negative during an injection, can be used to generate a larger stroke from the actuator than would otherwise be possible.
However, de-energise to inject systems suffer the disadvantage that for the majority (approximately 95%) of its operating life, i.e. while the injector is not injecting, the piezoelectric actuator stack must be maintained at a high positive differential voltage (e.g. 200 V). If moisture, for example, is present within the actuator, this high positive voltage can cause electrochemical degradation of the piezoelectric material, resulting, after time, in a short circuit failure and, hence, a reduced effective operational lifespan.
By contrast, an energise to inject injector (such as that described in EP 1555427) does not suffer as badly from electrochemical degradation, because it is only held at a high positive voltage during the 5% or so of time that it spends injecting. This type of prior art injector, however, cannot be run with significant bipolar voltages, since maintaining a negative voltage on the piezoelectric actuator for 95% of its operational life would result in actuator depolarisation, causing high electrical losses plus reduced actuator life and performance. Accordingly, prior art energise to inject piezoelectric-actuated fuel injectors do not so readily allow for large actuator displacements, which can be particularly disadvantageous where large fuel injections are required.
Another factor to consider is that prior art piezoelectric injectors require a relatively large and expensive piezoelectric actuator to provide the energy needed to lift the needle. Coupled with the fact that the amount of needle lift is limited by the capabilities of the actuator (even if a hydraulic amplifier is used to try to alleviate this problem), and any injector drive system limitations; the loss of the bipolar mode of operation in prior art energise to inject injectors severely limits the effectiveness of these injectors, particularly as injector nozzle flow requirements and fuel pressures increase.
Hence, there is a need for a fuel injector and a method of operating a fuel injector, in particular an energise to inject injector, which can enable the beneficial large actuator displacements achieved in de-energise to inject injectors, while reducing the possibility of any undesirable reductions in actuator response and lifespan.
This invention relates to a method for operating a piezoelectric energise to inject injector so as to overcome or at least alleviate at least one of the above-mentioned problems in the prior art.